The influence of incorporating different sizes of Si 3 N 4 particles on the microstructure and corrosion properties of a phosphate-based plasma electrolytic oxidation (PEO) coating on AM50 magnesium alloy was investigated. The experiments for this study were performed in alkaline electrolytes containing 1 g/L KOH, 10 g/L Na 3 PO 4 with and without three different sized of Si 3 N 4 particles. The corrosion properties of PEO coatings were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.5 wt.% NaCl solution. Microstructure observations by SEM showed that the surface morphology and composition of the PEO coating were affected greatly by particle addition. Si 3 N 4 particles can still be found without decomposition in the final coating due to their high melting point.